We apply automata learning techniques to learn fragments of the TCP network protocol by observing its external behaviour. We show that different implementations of TCP in Windows 8 and Ubuntu induce different automata models, thus allowing for fingerprinting of these implementations. In order to infer our models we use the notion of a mapper component introduced by Aarts, Jonsson and Uijen, which abstracts the large number of possible TCP packets into a limited number of abstract actions that can be handled by the regular inference tool LearnLib. Inspection of the learned models reveals that both Windows 8 and Ubuntu 13.10 violate RFC 793.